I just needed a device that would render my documents properly. I have mainly PDFs and some EPUBs. It was quite disappointing on the day I got my M91S to find two PDF books that had parts that don't render properly on the M91S. Even the Kindle DX, which I thought had poor PDF support, renders them correctly, although it has problems that the M91S doesn't have.
On the other hand, I have a large scanned PDF (basically each page is just an image from the scan) that's 300MB, and the M91S seems to render it fine and quickly enough, whereas the Kindle DX is so slow I could probably make tea while the page is turning.